Orders over $75 ship free using a mix of UPS Ground and SurePost. Your order will arrive within 2-7 days, depending on your vicinity to Minneapolis. Place your order by 3:30 PM CST Monday through Friday and we'll ship it the same day. This offer is only available to the contiguous US. USPS shipments do not qualify for free shipping.

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Select our $4.99 flat-rate shipping option, and your order will arrive to your door within 3 business days or less in the contiguous US. Place your order by 3:30 PM CST Monday through Friday and we'll ship it the same day. See map below for transit times.

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Free economy shipping over $75

Orders over $75 ship free using a mix of UPS Ground and SurePost. Your order will arrive within 2-7 days, depending on your vicinity to Minneapolis. Place your order by 3:30 PM CST Monday through Friday and we'll ship it the same day. This offer is only available to the contiguous US. USPS shipments do not qualify for free shipping.

Fast flat-rate shipping $4.99

Select our $4.99 flat-rate shipping option, and your order will arrive to your door within 3 business days or less in the contiguous US. Place your order by 3:30 PM CST Monday through Friday and we'll ship it the same day. See map below for transit times.

Quick Overview

One of the safest ATO’s available. Utilizes an electronic eye sensor to monitor water levels and a backup float switch sensor with audible alarm. A controller protects the quiet metering pump from running dry and reports an empty storage tank. Two magnetic holders for sensors and multiple mounting brackets included as well as a pump.

Excellent ATO SystemBy 40g Longon 7/22/2015

40g Long would recommend this product to a friend

After we finally got our tank set-up, I was amazed at the amount of evaporation from day to day. Being new to the hobby, I was concerned with maintaining stable water parameters. . I watched all the videos, researched the high tech models to the DIY versions, and decided the Tunze would be the ATO for our system. Since it's installation it has been performed flawless. I highly recommend this product.

great productBy rrosenbon 7/22/2015

rrosenb would recommend this product to a friend

So easy to install and works perfectly. I wish all aquarium products could be more like this

Excellent ATOBy Connyon 7/22/2015

Conny would recommend this product to a friend

Well built, easy setup, reliable- excellent ATO

Works greatBy Daveon 7/17/2015

Dave would recommend this product to a friend

This thing works great!! A little pricy compared to other top off brands but very dependable and unlikely to fail. I would recommend this product over all the other ones on the market. Easy to install too!
Dave
Birmingham

Solid and reliableBy Steveon 7/16/2015

Steve would recommend this product to a friend

The Tunze 3155 Auto Top Off system is the best there is. This is my second 3155 system, just installed on my new 185 gal tank build. My first 3155 system (on my first,180 gal, tank build) continues to function perfectly, with no issues, after more than 3 years of service.

Nice simple and reliableBy JEMM55on 7/14/2015

JEMM55 would recommend this product to a friend

Better than the rest...the electric eye beats the mechanicals hands down

Loving my Tunze ATO 3155By A.J.on 7/13/2015

A.J. would recommend this product to a friend

After a fair amount of research and reading many customer reviews I pulled the trigger and bought the 3155. Works perfect and only took a few min to set up. It really take the stress out of traveling, not having to worry about your water level. ALSO - has kept my salinity rock solid for over 3 months now, which my corals are loving!!

great customer serviceBy dozer42on 6/20/2015

dozer42 would recommend this product to a friend

My ato quit working. Contacted tunze and they had me send it in. Long story short my pump went bad and burned out the controller unit. I had to buy a new pump but even though my controller unit was out of warranty they replaced it for free.

Must HaveBy RICHARDon 6/16/2015

RICHARD would recommend this product to a friend

This works great and the two different sensors are awesome. I even plumbed this to feed my two foot tall DIY kalk reactor and it feeds the reactor with no issues.

Get it. By Michaelon 6/12/2015

Michael would recommend this product to a friend

This is my first and probably the will be the last ATO system I will need to buy...unless I set up another reef tank of course. Forget about the "hefty" price tag, your tank is certainly worth it.

Excellent product!By Josephon 6/1/2015

Joseph would recommend this product to a friend

I have this setup currently for my system. While the instructions are not as straightforward as I'd like, there are plenty of videos online to learn how to set everything up. There are several ways to use this product which are part of what makes it good but also part of why the instructions are a little confusing. Nevertheless, it works great and worth the money IMO. Get it.

EXCELLENT PRODUCTBy MARIOon 6/1/2015

MARIO would recommend this product to a friend

IT IS A PEACE OF MIND, NOT HAVING TO WORRYING IN ADDING WATER AND SALT EVERY TIME IT GETS LOW.

Product Questions

I have a litermeter 3 that has failed. It pushes water about 30 feet from the litermeter to a sump tank. Will this pump do the same?

Question by: Spencer on Apr 29, 2014 12:54:00 PM

Hi Spencer,
The max head pressure on the included pump is about 7ft. It likely wouldn't be well suited to this application. That said though, you can get the additional switch socket adapter which would allow you to control any pump via the osmolator system. That way you can use a pump that can handle the head pressure.

If i have a wave maker and this ATO in the same tank (I don't have a sump), would the waves cause the pump to turn on and off constantly? Thanks

Question by: Christian on Jul 17, 2014 9:40:00 PM

Hi Christian,
In theory it could present a problem but the circuitry in the Osmolator should help to prevent that. Every time the ATO is triggered it runs for a minimum of 15 seconds. It adds overfill to prevent short cycling. In most cases this should be sufficient unless the height of your waves is constantly changing.

Hi Jim,
The Tunze Osmolator is an entirely self sufficient system. It is perfectly fine to connect the Osmolator to one of the Apex's power outlets incase you wanted to cut the power for some reason (like when you are doing a water change) though the Apex isn't capable of reading the Osmolator's sensors or anything of that sort.

Hello,
Just wondering if I need 2 ATO. I know you can place it either in the sump or in the tank, but my question is if I place it in the sump will it also refill my tank in that process. I don't see that happening but if it does please let me know.

Question by: Ronald on Jan 13, 2014 2:32:00 AM

Hi Ronald,
If you have a sump, it MUST go in the sump, in the chamber where your return pump is. This is the only area of your tank that will have a changing water level depending on evaporation (unless its far too late!). Only one ATO would be necessary but it must go in that location if you have a sump.

My Tunze Osmolator ATO keeps getting stuck in the ON position. I soaked the electronic eye in vinegar and it was fine for a week, but then it stuck on again. Do these electronic eyes fail after many years of use? I've been using it for about 4 years. Is there any way to replace the eye without having to buy a whole new system?

Thanks,
Daire Fontaine

Question by: Daire Fontaine on Jan 17, 2014 12:23:00 AM

Hi Daire,
We run into very few issues with these but after that length of time I would imagine its possible for any component to fail. We don't have individual parts like the optical eye, but if you give Roger and company a shout over at Tunze USA, he might. At the very least he could likely help you narrow down exactly what it is. The number I have for them is 512-833-7546 tunze@sbcglobal.net

Does the pump get placed directly in the top off water reservoir? If so what are the dimensions of that pump? I am trying to decide what kind of reservoir I can use before purchasing a ATO.

Question by: Mark on Jun 5, 2014 8:39:00 AM

Hey Mark,
That is correct. Its a submersible pump that gets stuck right into the top off water. If your interested in this ATO system you should give this blog article a quick read. It goes into a lot more detail on the system (which is one of my favorites).

Bulk Reef Supply tried many different water level switches and auto top off solutions before giving the Tunze 3155 Osmolater Universal Auto Top Off BRS Recommended status. Despite its higher price this is by far our favorite solution. Spending the extra money on a Tunze Osmolater will give you that extra peace of mind and insurance from having a disaster that ruins carpets or hard wood floors.

Ask a question

Product Questions

I have a litermeter 3 that has failed. It pushes water about 30 feet from the litermeter to a sump tank. Will this pump do the same?

Question by: Spencer on Apr 29, 2014 12:54:00 PM

Hi Spencer,
The max head pressure on the included pump is about 7ft. It likely wouldn't be well suited to this application. That said though, you can get the additional switch socket adapter which would allow you to control any pump via the osmolator system. That way you can use a pump that can handle the head pressure.

If i have a wave maker and this ATO in the same tank (I don't have a sump), would the waves cause the pump to turn on and off constantly? Thanks

Question by: Christian on Jul 17, 2014 9:40:00 PM

Hi Christian,
In theory it could present a problem but the circuitry in the Osmolator should help to prevent that. Every time the ATO is triggered it runs for a minimum of 15 seconds. It adds overfill to prevent short cycling. In most cases this should be sufficient unless the height of your waves is constantly changing.

Hi Jim,
The Tunze Osmolator is an entirely self sufficient system. It is perfectly fine to connect the Osmolator to one of the Apex's power outlets incase you wanted to cut the power for some reason (like when you are doing a water change) though the Apex isn't capable of reading the Osmolator's sensors or anything of that sort.

Hello,
Just wondering if I need 2 ATO. I know you can place it either in the sump or in the tank, but my question is if I place it in the sump will it also refill my tank in that process. I don't see that happening but if it does please let me know.

Question by: Ronald on Jan 13, 2014 2:32:00 AM

Hi Ronald,
If you have a sump, it MUST go in the sump, in the chamber where your return pump is. This is the only area of your tank that will have a changing water level depending on evaporation (unless its far too late!). Only one ATO would be necessary but it must go in that location if you have a sump.

My Tunze Osmolator ATO keeps getting stuck in the ON position. I soaked the electronic eye in vinegar and it was fine for a week, but then it stuck on again. Do these electronic eyes fail after many years of use? I've been using it for about 4 years. Is there any way to replace the eye without having to buy a whole new system?

Thanks,
Daire Fontaine

Question by: Daire Fontaine on Jan 17, 2014 12:23:00 AM

Hi Daire,
We run into very few issues with these but after that length of time I would imagine its possible for any component to fail. We don't have individual parts like the optical eye, but if you give Roger and company a shout over at Tunze USA, he might. At the very least he could likely help you narrow down exactly what it is. The number I have for them is 512-833-7546 tunze@sbcglobal.net

Does the pump get placed directly in the top off water reservoir? If so what are the dimensions of that pump? I am trying to decide what kind of reservoir I can use before purchasing a ATO.

Question by: Mark on Jun 5, 2014 8:39:00 AM

Hey Mark,
That is correct. Its a submersible pump that gets stuck right into the top off water. If your interested in this ATO system you should give this blog article a quick read. It goes into a lot more detail on the system (which is one of my favorites).

Full Details

Bulk Reef Supply tried many different water level switches and auto top off solutions before giving the Tunze 3155 Osmolater Universal Auto Top Off BRS Recommended status. Despite its higher price this is by far our favorite solution. Spending the extra money on a Tunze Osmolater will give you that extra peace of mind and insurance from having a disaster that ruins carpets or hard wood floors.

Reviews

Excellent ATO SystemBy 40g Longon 7/22/2015

40g Long would recommend this product to a friend

After we finally got our tank set-up, I was amazed at the amount of evaporation from day to day. Being new to the hobby, I was concerned with maintaining stable water parameters. . I watched all the videos, researched the high tech models to the DIY versions, and decided the Tunze would be the ATO for our system. Since it's installation it has been performed flawless. I highly recommend this product.

great productBy rrosenbon 7/22/2015

rrosenb would recommend this product to a friend

So easy to install and works perfectly. I wish all aquarium products could be more like this

Excellent ATOBy Connyon 7/22/2015

Conny would recommend this product to a friend

Well built, easy setup, reliable- excellent ATO

Works greatBy Daveon 7/17/2015

Dave would recommend this product to a friend

This thing works great!! A little pricy compared to other top off brands but very dependable and unlikely to fail. I would recommend this product over all the other ones on the market. Easy to install too!
Dave
Birmingham

Solid and reliableBy Steveon 7/16/2015

Steve would recommend this product to a friend

The Tunze 3155 Auto Top Off system is the best there is. This is my second 3155 system, just installed on my new 185 gal tank build. My first 3155 system (on my first,180 gal, tank build) continues to function perfectly, with no issues, after more than 3 years of service.

Nice simple and reliableBy JEMM55on 7/14/2015

JEMM55 would recommend this product to a friend

Better than the rest...the electric eye beats the mechanicals hands down

Loving my Tunze ATO 3155By A.J.on 7/13/2015

A.J. would recommend this product to a friend

After a fair amount of research and reading many customer reviews I pulled the trigger and bought the 3155. Works perfect and only took a few min to set up. It really take the stress out of traveling, not having to worry about your water level. ALSO - has kept my salinity rock solid for over 3 months now, which my corals are loving!!

great customer serviceBy dozer42on 6/20/2015

dozer42 would recommend this product to a friend

My ato quit working. Contacted tunze and they had me send it in. Long story short my pump went bad and burned out the controller unit. I had to buy a new pump but even though my controller unit was out of warranty they replaced it for free.

Must HaveBy RICHARDon 6/16/2015

RICHARD would recommend this product to a friend

This works great and the two different sensors are awesome. I even plumbed this to feed my two foot tall DIY kalk reactor and it feeds the reactor with no issues.

Get it. By Michaelon 6/12/2015

Michael would recommend this product to a friend

This is my first and probably the will be the last ATO system I will need to buy...unless I set up another reef tank of course. Forget about the "hefty" price tag, your tank is certainly worth it.

Excellent product!By Josephon 6/1/2015

Joseph would recommend this product to a friend

I have this setup currently for my system. While the instructions are not as straightforward as I'd like, there are plenty of videos online to learn how to set everything up. There are several ways to use this product which are part of what makes it good but also part of why the instructions are a little confusing. Nevertheless, it works great and worth the money IMO. Get it.

EXCELLENT PRODUCTBy MARIOon 6/1/2015

MARIO would recommend this product to a friend

IT IS A PEACE OF MIND, NOT HAVING TO WORRYING IN ADDING WATER AND SALT EVERY TIME IT GETS LOW.

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